The unique properties of nickel-titanium alloys

1. Shape memory property (shape memory) Shape memory refers to the phenomenon where, when a certain shape of the parent phase is cooled from above the Af temperature to below the Mf temperature to form martensite, the martensite is deformed at a temperature below Mf, and after heating to below the Af temperature, the reverse phase transformation occurs, and the material will automatically restore its shape in the parent phase. In fact, the shape memory effect is a thermally-induced phase transformation process of nickel-titanium alloy.

2. Super elasticity (super elastic) The so-called super elasticity refers to the phenomenon where the sample generates a strain much larger than the elastic limit under the action of an external force, and the strain can be automatically restored when the force is removed. That is, in the parent phase state, due to the action of the external stress, the stress-induced martensite phase transformation occurs, and the alloy exhibits mechanical behavior different from that of ordinary materials. Its elastic limit is much greater than that of ordinary materials, and it no longer follows Hooke's law. Compared with the shape memory property, super elasticity does not involve heat. In summary, super elasticity refers to the situation where the stress does not increase with the increase of strain within a certain deformation range. Clinically, it is manifested as the orthodontic force generated during the deformation of the archwire remaining constant and not gradually decreasing as the teeth move towards the orthodontic direction. According to the characteristics of the stress-strain curve corresponding to super elasticity, super elasticity can be divided into two types: linear super elasticity and non-linear super elasticity. The stress-strain curve of the former has a nearly linear relationship between stress and strain. Non-linear super elasticity refers to the result of stress-induced martensite phase transformation and its reverse phase transformation during loading and unloading within a certain temperature range above Af. Therefore, non-linear super elasticity is also called pseudo elasticity of phase transformation. The pseudo elasticity of nickel-titanium alloy can reach about 8%. The super elasticity of nickel-titanium alloy can change with the conditions of heat treatment. When the archwire is heated above 400°C, the super elasticity begins to decrease. When the heat treatment temperature exceeds 600°C, the super elasticity basically disappears. Based on this characteristic, in clinical practice, the non-orthodontic area of the archwire can be subjected to heat treatment to lose super elasticity, so as to avoid the influence of the orthodontic process on the non-orthodontic area of the teeth, while the archwire in the orthodontic area still has good elasticity.

3. Sensitivity to oral temperature changes: Stainless steel wires and CoCr alloy orthodontic wires have a force that is basically not affected by the oral temperature. The force of super elastic nickel-titanium alloy orthodontic wires changes with the change of oral temperature. When the deformation is constant, the temperature increases, and the force increases. On the one hand, it can accelerate the movement of teeth because the temperature change in the oral cavity will stimulate the blood flow stagnation at the sites where capillary stasis is caused by orthodontic devices, thereby allowing the repair cells to receive sufficient nutrition and maintain their vitality and normal function during the movement of teeth. On the other hand, orthodontists cannot precisely control or measure the force in the oral environment.

4. Corrosion resistance: Studies have shown that the corrosion resistance of nickel-titanium wires is similar to that of stainless steel wires.

5. Anti-toxicity: The special chemical composition of nickel-titanium shape memory alloy, that is, it is a nickel-titanium atomic alloy, contains about 50% nickel, and it is known that nickel has carcinogenic and carcinogenic effects. Generally, the surface layer of titanium oxide acts as a barrier, making the Ni-Ti alloy have good biocompatibility. The TiXOy and TixNiOy on the surface layer can inhibit the release of Ni.

6. Gentle orthodontic force: Currently, commercially available orthodontic metal wires include austenitic stainless steel wires, cobalt-chromium-nickel alloy wires, nickel-chromium alloy wires, Australian alloy wires, gold alloy wires, and titanium alloy wires. Regarding the load-displacement curves under the conditions of tensile test and three-point bending test for these orthodontic correction metal wires. The unloading curve platform of nickel-titanium alloy is the lowest and the flattest, indicating that it can provide the most durable and gentle orthodontic force.

7. Excellent shock absorption properties: Since the greater the vibration caused by chewing and nocturnal bruxism to the archwire, the greater the damage to the tooth roots and periodontal tissues. Through the results of different archwire attenuation experiments, it was found that the vibration amplitude of stainless steel wire is larger than that of super elastic nickel-titanium wire. The initial vibration amplitude of super elastic nickel-titanium archwire is only half that of stainless steel wire. The good vibration and shock absorption properties of the archwire are very important for the health of teeth, while traditional archwires such as stainless steel wire tend to aggravate the absorption of tooth roots.